1. Field of the Invention
This invention generally relates to a system and method of controlling a weld current for use in arc welding processes, and more particularly to a software-based system and method which permits generation of a wide variety of weld current profiles, including weld current profiles that can dynamically adapt to changes in welding conditions simply by changing the timing of segments of the weld current profile having predetermined (but not necessarily constant) amplitude profiles.
Unlike prior systems utilizing waveform generation to control a weld current power supply, the waveform generation system and method of the invention requires neither conventional hardware-implemented arc current or voltage feedback loops, nor complex digital signal processor-based controllers.
The reference waveforms used to generate the weld current profiles may be in the form of tables containing amplitude and trigger or duration values for each segment of the waveform. The waveforms may either be applied without feedback directly to the weld current power supply, or digitally modified based on feedback from an arc current and/or voltage sensor. Modification of the profiles is carried out by modifying a trigger or duration parameter for a selected amplitude segment in the table used to represent the reference waveform, preferably based on current and/or voltage data gathered over multiple weld cycles, simplifying programming and reducing required processor resources.
Despite its simplicity, the software-based weld current control of the invention can carry-out, without limitation, any of the following relatively complex weld-current control functions:
in short-arc welding, synchronizing of the waveform to the start of a short circuit,
in short-arc welding, re-starting the short circuit portion of the waveform if premature re-arcing occurs and/or if the re-establishment of another short circuit is detected, indicating that the original short circuit was only an incipient short,
in short-arc welding, adjusting the duration of the short-circuit phase based on analysis of previous cycles, including adjustment to prevent stub-outs if re-arcing fails to occur,
in any consumable electrode process, generating a current amplitude profile for each of a plurality of electrode diameters, feed-rates, material types and welding gas types,
in any consumable electrode process, interpolating between previously generated current amplitude profiles for different electrode feed-rates to facilitate operation at electrode feed-rates other than those associated with a particular amplitude profile or table, and
in any pulsed arc process, using the software to also provide synchronized sensing and control of the torch-to-workpiece voltage by adjusting the torch-to-workpiece distance or, in pulsed arc consumable electrode processes only, using the software to provide synchronized sensing and control the torch-to-workpiece voltage by adjusting the duration of any one of the current amplitude segments.
It will of course be appreciated by those skilled in the art that the system and method of the invention may be applied to welding processes other than those specified above, including all types of manual, semiautomatic, and automatic welding, as well as all types of out-of-position and pipe welding processes.
2. Description of Related Art
In recent years, substantial effort has been devoted to improving control of weld currents in arc welding systems. Until recently, most of this effort has involved design of ever more sophisticated feedback circuits which modulate the output of the weld current generator in response to changes in currents or voltages measured between the electrode and the workpiece. With the development of improved microprocessors, however, attempts have been made to digitize many welding control functions, including control of the arc welding current. Transfer of hardware-implemented functions to software has the advantage of enabling a particular apparatus to be adapted for a greater variety of different welding processes or conditions, without the need to design specific circuits for each change in the welding sequence.
U.S. Pat. No. 4,973,814 illustrates a microprocessor-based weld current controller which, like the present invention, uses pre-determined reference waveforms as a basis for control of a weld current power supply. However, the output of the waveform generator described in this patent is applied as the command signal to a current amplitude feedback loop of the welding apparatus, rather than directly to the power supply, the current amplitude feedback loop including a comparator for comparing the applied waveform to the actual waveform and generating an error signal. The only adjustments made to the waveform itself are made through a display of the difference between the reference and actual waveforms, and a keyboard for adjusting the applied reference waveform as necessary to achieve a desired actual waveform.
In the system illustrated in U.S. Pat. No. 4,973,814, only the amplitude and not the duration of the various parts of the weld current cycle are dynamically adjusted, and the adjustment is carried out by an analog servo, rather than by modifying the reference waveform. Adjustment of the reference waveform can only be carried out manually. As a result, while systems of the type illustrated in this patent can, through the analog servo, provide automatic control of a relatively simple and stable weld cycle, once the appropriate waveform has been manually adjusted, they cannot be used to provide automatic control for welding processes that are highly dependent on the timing of events that occur during the cycle, such as short-circuit arc welding, in which the cycle consists of at least two segments of variable length.
U.S. Pat. No. 5,278,390, on the other hand, discloses a version of the controller disclosed in U.S. Pat. No. 4,973,814, in which the hardware-implemented closed loop servo function is transferred to a digital signal processor capable of modifying the reference xe2x80x9cwaveformxe2x80x9d expressed in a xe2x80x9cstate table.xe2x80x9d In the system disclosed in this patent, the current profile is stored in state tables that can be selected for a particular weld process. Feedback is achieved, as explained in col. 13, lines 13 et seq. of U.S. Pat. No. 5,278,390, by manipulating the current amplitude values contained in the active state table, resulting in a modified waveform which is applied to the output of a weld current power source.
The system disclosed in U.S. Pat. No. 5,278,390, which takes what may be referred to as a xe2x80x9cbrute forcexe2x80x9d approach to feedback, appears to be capable of controlling a wide variety of welding processes, but suffers from the disadvantages that the response to current or voltage feedback values must be separately programmed for each part of the current cycle, and the closed loop control circuit must be adaptable enough to recognize and respond appropriately to each different part of the weld cycle by continually adjusting the current amplitude values according to the different requirements for each portion of the weld current cycle. In a highly variable environment such a short-circuit arc welding system, without the assistance of hardware-based feedback loops, the current amplitude control circuit of U.S. Pat. No. 5,278,390 would essentially be required to re-write the state table during each cycle, with the weld current profile essentially being determined by programming of the controller rather than of the state table.
The present invention, in contrast, does not attempt to instantaneously modify a waveform to conform to changes in sensed arc voltage or current, but rather takes a simpler and yet more universally adaptable approach in which the waveform is divided into predetermined amplitude segments, and only the timing of the segments is varied, thereby eliminating the need to continually adjust amplitude values for each weld cycle.
In a sense, the invention is based on recognition that, for cyclical welding processes, differences in amplitude between a desired current profile and an actual profile result from differences in the timing of parts of the cycle. For example, in a short-circuit arc welding cycle, the establishment of a short circuit is followed by an increase in the weld current. If the short-circuit is established early, a control circuit of the type disclosed in U.S. Pat. No. 5,278,390 would need to almost instantaneously determine that a short-circuit had been established and change all of the corresponding amplitude values in the state table accordingly. The system and method of the invention, on the other hand, can respond to establishment of the short-circuit simply by triggering a short circuit segment of the current profile, without having to change current amplitude values, which are presumed to be the same from one short-circuit cycle to the next. A more complex short-circuit current profile having six or more segments, such as the ones described in U.S. Pat. Nos. 4,544,826; 4,546,234; 4,717,807; 4,835,360; 4,866,247; 4,897,523; 4,954,691; 4,972,064; 5,003,154; 5,148,001; and 5,961,863, would be very difficult to implement using adjustment of state table current amplitude values, whereas a programmer utilizing the present invention would simply need to select an appropriate current amplitude profile, and program appropriate triggers for any duration variable segments in the profile.
As evidence of the intractability of the state-table approach taken in U.S. Pat. No. 5,278,390, a later patent by the same assignee, U.S. Pat. No. 6,002,104, essentially abandons the concept of using software to achieve automatic real-time control of a complex waveform, such as the waveform required to minimize splatter during short-circuit arc welding, and instead utilizes a sophisticated xe2x80x9cmicroprocessor based real-time control and monitor to enable real time manual adjustment of a short circuit arc welding current,xe2x80x9d i.e., to enable a human controller to adjust the welding current during the welding process. Other systems that provide for real-time manual adjustment of welding parameters are disclosed in U.S. Pat. Nos. 5,571,431, 4,390,954, and 4,189,765.
Besides U.S. Pat. No. 5,278,390, the only other patent known to the inventors that is directed to software-based control of a welding arc using a periodic reference waveform without the need for manual intervention is U.S. Pat. No. 4,650,957. However, the reference waveform described in this patent is not applied to the weld-current power supply, but rather is applied to a torch-to-workpiece distance controller, and thus has limited applicability. While numerous other software-based arc voltage or current controllers have previously been disclosed, including the one described in U.S. Pat. No. 3,838,244, which dates back to 1968, none of the controllers described in these patents use periodic reference waveforms, much less a table-driven waveform generator, to control the weld current. Examples of such patents include U.S. Pat. Nos. 4,019,016, 3,838,244, and 3,689,734, which disclose digital controllers that provide pre-programmed digital control of current or arc voltage levels in synchronism with movement of a welder along a carriage and a track, U.S. Pat. No. 4,561,059, which is directed to digital control of welding power supply to achieve constant current. While a constant-current controller might use software to adjust the weld current, there is no need to initially generate a waveform or to adjust the duration of portions of the waveform based on detected currents.
In summary, none of the patents discussed above discloses the concept of controlling a weld-current by applying to the weld-current power supply a current profile consisting of a time sequence of amplitude segments, and of using software to adjust the weld-current solely by modifying the timing of individual segments in the current profile, without adjusting amplitudes of the individual segments, based on feedback of the actual weld-current (or voltage or power/energy), whether in real time or based on data obtained from previous cycles. Previous waveform-based approaches to automatic weld current control, such as the ones described in U.S. Pat. Nos. 4,973,814 and 5,278,390, have relied on application of a fixed but manually adjustable waveform to an analog hardware-implemented current amplitude servo, or a state table approach that requires continuous adjustment of current amplitude values in the table, while other waveform-based approaches, such as the one described in U.S. Pat. No. 6,002,104, do not even attempt to achieve automatic control, but rather rely entirely on operator intervention to adjust the waveform.
It is accordingly a first objective of the invention to overcome the disadvantages of prior arc welding current control systems and methods by providing a software-based arc welding current control that is easily adapted to a wide variety of welding processes, that can automatically adjust for changes in weld conditions, and yet that does not require high level programming skill and extraordinary processing power to implement.
It is a second objective of the invention to provide a software-based weld current controller that can adapt to complex, highly variable waveforms, and yet does not require operation intervention during welding.
It is a third objective of the invention to provide a more flexible, easily adapted arc welding system that can be adapted to generate complex arc welding waveforms without the need for specific feedback circuitry to control the waveforms, and yet that can also economically be adapted to generate less complex waveforms.
It is a fourth objective of the invention to provide arc welding waveform generation apparatus that can be adapted for use with short-arc, pulsed arc, and other types of consumable and non-consumable arc welding processes.
It is a fifth objective of the invention to provide waveform generation apparatus that can be adapted for use with conventional rapid-response welding power supplies to provide improved short-arc welding characteristics without the need for necking detection or the need to provide additional costly high-power switching elements to facilitate rapid current decay.
These objectives are achieved, in accordance with the principles of a preferred embodiment of the invention, by providing a system and method of controlling a weld current through the use of reference waveforms applied to the weld current power supply, and that may be dynamically modified based on current or voltage feedback to the waveform generator. Unlike prior systems utilizing waveform generation to control a weld current power supply, the waveform generation system and method of the invention requires neither conventional hardware-implemented arc current or voltage feedback loops, nor complex digital signal processor-based controllers.
The reference waveforms used to generate the weld current profiles may be in the form of tables containing amplitude and trigger or duration values for each segment of the waveform. The waveforms may either be applied without feedback directly to the weld current power supply, or digitally adjusted based on feedback from an arc current and/or voltage sensor. Adjustment of the profiles is carried out by modifying a trigger or duration parameter for a selected amplitude segment in the table used to represent the reference waveform, preferably based on current and/or voltage data gathered over multiple weld cycles, in order to simplify programming and reducing required processor resources.
In addition, according to preferred embodiments of the invention, the system and method provides for the following specific software adjustments or modification of the waveform in response to detection of the arc current or voltage:
in short-arc welding, synchronization of the waveform to the start of short circuit,
in short-arc welding, re-starting the short circuit portion of the waveform if premature re-arcing occurs or if re-establishment of a short circuit is detected, indicating that the short circuit was only an incipient short,
in short-arc welding, adjusting the duration of the short-circuit phase based on analysis of previous cycles, including adjustment to prevent stub-outs if re-arcing fails to occur,
in any consumable electrode process, generating current amplitude profiles for each of a plurality of electrode diameters, feed-rates, material types and welding gas types,
in any consumable electrode process, interpolating between previously generated current amplitude profiles for different electrode feed-rates to facilitate operation at other electrode feed-rates, and
in any pulsed arc process, using the software to also provide synchronized sensing and control of the torch-to-workpiece voltage by adjusting the torch-to-workpiece distance or, in pulsed arc consumable electrode processes only, using the software to provide synchronized sensing and control the torch-to-workpiece voltage by adjusting the duration of any one of the current amplitude segments.